Relationship quality, sex and hurricane exposure predict white blood cell counts in a free-ranging primate population

Abstract Social relationships and environmental disturbances both shape health and survival, yet the immune processes linking sociality to health remain poorly understood, particularly in populations experiencing ecological disruption. Here, we examine how social relationships, demographic factors and environmental disturbance relate to variation in white blood cell (leukocyte) composition in free-ranging rhesus macaques (Macaca mulatta). First, we tested whether relationship quality (weak and strong affiliative ties), social status, sex and age were associated with three leukocyte measures: the neutrophil-to-lymphocyte ratio, neutrophil count and eosinophil count. We found opposing sex-specific effects on eosinophil counts. Females with more weak social ties had lower counts whereas males with stronger ties had higher counts. We then used opportunistic cross-sectional data to evaluate how a major natural disaster, Hurricane Maria (in 2017), influenced these immune measures and their link to sociality. Individuals exposed to the hurricane showed elevated leukocyte counts across all three measures but neither social status nor affiliative relationships moderated these effects. Together, these findings document sex-specific links between social structure and immune cell composition, alongside broad immune alterations associated with ecological disturbance. Longitudinal studies will be essential to disentangle causal mechanisms and to understand how social and environmental factors jointly shape immunity in natural populations.